The present invention relates to multi-echo imaging method and apparatus utilizing nuclear magnetic resonance (hereinafter simply referred to as "NHR") and more particularly to imaging method and apparatus capable of making correcter an image obtained in respect of spin-spin relaxation time T2.
Conventionally, the multi-echo imaging method is described in, for example, "Magnetic Resonance in Medicine, 3, pp 397-417 (1986)".
When a 180.degree. RF pulse is applied to a magnetization vector M thrown down by a 90.degree. RF pulse to a predetermined plane, the resulting vector has, in addition to a component M- which is 180.degree. rotated as depigned, a component M+ not rotated and a component Mo rotated by half the angle (=90.degree.). The components M+ and Mo develop because the 180.degree. RF pulse is not generated correctly and the magnetization vector M is not rotated through exact 180.degree. (see U.S. Pat. No. 4,484,138). Of the unwanted components, Mo is directed in the same direction as that of the static magnetic field and is therefore not detected as an echo signal representative of T2. On the other hand, the component M+ forms an artifact.
As is known in the art, the component M+ leading to the artifact can be dephased effectively by applying pulses of additional magnetic field gradient before and after the 180.degree. RF pulse. In the aforementioned U.S. Pat. No. 4,484,138, the additional pulse is called a crusher pulse.
Exactly, the artifact can be cancelled out by the additional pulses. However, an image is formed by putting together a plurality of echoes, that is, by applying the 180.degree. RF pulse a plurality of times, the resulting image becomes incorrect. This is because the echo signals sampled for formation of the image contain error signals.
For example, when considering an instance where the 180.degree. RF pulse is applied three times, a preferable component is rotated through 180.degree..times.3=540.degree. (=180.degree.). Contrary to this, many components causing error signals (error components) such as of 0.degree.+180.degree.+0.degree. and 90.degree.+0.degree.+90.degree. exist.
The greater the number of applications of the 180.degree. RF pulse, the greater the number of error components becomes.